Process furnaces such as slot furnaces are well-known for such purposes as forging ferrous materials. Such furnaces often have an elongated horizontal slot through which the material to be heated is thrust in and taken out. The furnaces often operate with internal wall temperatures of between 1500.degree. F and 2600.degree. F or higher. Heat loss by radiation from the slot at such high temperatures represents a large fraction of the heat supplied to the furnace. Iron and similar doors have often been used to close slots in the prior art but such materials absorb and reradiate heat to the outside affording little control over heat loss by radiation.
The radiation loss through the slot can be significant depending of course on the operating temperature, door area and perhaps other parameters. The iron door closure for conventional slot furnaces does not avoid the radiation loss problem.